study of frequency modulation and demodulation using voice link

7/27/2019 study of frequency modulation and demodulation using voice link

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STUDY OF FREQUENCY MODULATION & DEMODULATION USING VOICE

LINK.

OBJECTIVE:

The objective of this experiment is to study the circuit action of Frequency

Modulation & Demodulation over Fiber Optic Link using 660 nm and 950nm LED.

EQUIPMENTS:

Link-A kit. 20 MHz Dual Trace Oscilloscope. 1 Meter Fiber cable. Power supply.

THEORY:

FREQUENCY MODULATION: It is a type of modulation in which the frequency of

the high frequency (Carrier) is varied in accordance with the instantaneous valueof the modulating signal.MAIN ASPECTS: Consider a sine wave signal Vm(t) with pulse Vm(t) = B•sin(w•t) and another sine wave Vc(t) with upper Ω pulse: Vc(t) = A • sin(Ω•t) Thesignal Vm(t) is called modulating signal, the signal Vc(t) is called carrier signal.Vary the frequency of the carrier Vc(t) in a way proportional to the amplitude of the modulating signal Vm(t). You obtain a Vm(t) frequency modulated diagonal,which can be expressed by the relation: Vm(t) = A • sin [Θ(t)] with Θ(t)instantaneous angle function of Vm(t).

MATHEMATICAL EXPRESSION OF THE FREQUENCY MODULATED SIGNAL:The instantaneous pulse

Ω(t) of the FM signal by definition:

Ω(t) =

Ω+ K• Vm(t)

with hΩ = carrier pulse K= modulation sensitivityThe instantaneous angle Ω (t) to be used as subject of the sine to obtain themathematical operation of the FM signal, is detected by integrating Ω(t): Θ(t) =∫Ω(t) dt In the case of modulating sine wave signal [Vm (t) = B•sin (w•t)], Θ(t) itresults: Θ(t) = Ω•(t) –(K•B/w)•cos(w•t) The expression of the frequency modulated signal Vm (t)becomes: Vm(t) = A • sin[Ω• (t)- (K•B/w) • cos(w•t)]FREQUENCY DEVIATION ∆F AND MODULATION INDEX MF:The instantaneous frequency F(t) of the carrier modulated by a sine wave,results:F(t) = Ω(t)/2π = Ω/2π K•B•sin(w•t) and oscillates between a minimum Fmin anda maximum value Fmax: Fmin = Ω/2π K•B/2π Fmax = Ω/2π K•B/2π The frequency deviation ∆F represents the maximum shift between themodulated signal frequency, over and under the frequency of the carrier:∆F=(Fmax–Fmin)/2We define as modulation index mf the ratio between ∆F and the modulatingfrequency f: mf = ∆F/f



FREQUENCY MODULATION GENERATION: The circuits used to generate afrequency modulation must vary the frequency of a high frequency signal (carrier)as function of the amplitude of a low frequency signal (modulating signal). Inpractice there are two main methods used to generate the FM:DIRECT METHOD: An oscillator is used in which the reactance of one of the

elements of the resonant circuit depends on the modulating voltage. The mostcommon device with variable reactance is the Varactor or Varicap, which is aparticular diode which capacitance varies as function of the reverse bias voltage(the Varicap is described in the next chapter). The frequency of the carrier isestablished with AFC circuits (Automatic Frequency Control) or PLL (PhaseLocked Loop). In this KIT FM is generated by using FM generation IC. Thisgenerated FM is then sent via Fiber Optic and is received by optical detector.The output of the detector is then fed to the FM demodulation section. Thefrequency of the carrier is established with AFC circuits (Automatic FrequencyControl) or PLL(Phase Locked Loop).INDIRECT METHOD: The FM is obtained in this case by a Phase Modulation,

after the modulating signal as been integrated. In the phase modulator, thecarrier can be generated by a quartz oscillator, and so its frequency stabilizationis easier. In the circuit used for the exercise, the frequency modulation isgenerated by a Hartley oscillator, which frequency is determined by a fixedinductance and by a capacity (variable) supplied by Varicap diodes.FREQUENCY DEMODULATION: To demodulate a frequency modulated signala circuit is necessary which supplies the output with proportional voltage to thefrequency deviation of the input modulated signal. The ideal characteristic of thedemodulator is a straight line, also if it is actually sufficient to obtaincharacteristics which presents a linear behavior only for a certain frequencyrange (demodulator usage range).The figure reports: The instantaneous frequency f of the modulated signal,oscillating between F1 and F2 (Fc frequency of the carrier). Thevoltage/frequency characteristic curve of the demodulator.The detected signal.DEMODULATION OF FM SIGNALS: An FM receiver is very similar to an AMreceiver. The most significant change is that the demodulator must now extractthe information signal from a frequency, rather than an amplitude, modulatedwave.The basic requirement of any FM demodulator is therefore to convert frequencychanges into changes in voltage, with the minimum amount of distortion.To achieve this, it should ideally have a linear voltage/frequency characteristic.

A ‘demodulator’ can also be called a ‘discriminator’ or a ‘detector’. Any design of circuit that has a linear voltage/frequency characteristic would beacceptable and we are going to consider the five most popular types. In eachcase the main points to look for are: How do they convert FM signals into AMsignals? How linear is their response - this determines the amount of distortion inthe final output? How good are they at rejecting noise signals?FREQUENCY DEMODULATOR CIRCUITS: For the detection of the frequencymodulated signals different circuit solutions have been used, some are out of useand others are used at the moment. Among the first ones, we mention:



PLL DETECTOR: It constitutes one of the applications of the Phase Locked Loopand is less sensitive to noise.THE PHASE – LOCKED LOOP (PLL) DETECTOR: This is another demodulator that employs a phase comparator circuit. It is a very good demodulator and hasthe advantage that it is available as a self-contained integrated circuit so there is

no setting up required-you plug it in and it works. For these reasons it is oftenused in commercial broadcast receivers. It has very low levels of distortion and isalmost immune from external noise signals and provides very low levels of distortion. Altogether a very nice circuit.The overall action of the circuit may, at first, seem rather pointless. The incomingFM signal is taken to one input of the PHASE COMPARATOR CIRCUIT, whereits phase is compared with the square-wave output from the VOLTAGECONTROLLED OSCILLATOR (VCO). If the central frequency of the detector andthe carrier frequency of the FM signal and local oscillator frequency coincide, youwill get the original reproduction of the modulating signal. There is a voltage-controlled oscillator (VCO). The frequency of this oscillator is controlled by the

DC output voltage from the output of the low pass filter. Now, this DC voltagekeeps the oscillator running at the same frequency as the original input signaland 90° out of phase.The question often arises as to why we would want the oscillator to run at thesame frequency and 90° out of phase. And if we did, then why not just add aphase shifting circuit at the input to give the 90° phase shift?The answer can be seen by imagining what happens when the input frequencychanges – as it would with an FM signal. If the input frequency increases anddecreases, the VCO frequency is made to follow it. To do this, the input controlvoltage must increase and decrease. It is these changes of DC voltage level thatform the demodulated signal. The AM signal then passes through a signal buffer to prevent any loading effects from disturbing the VCO and then through anaudio amplifier it necessary. The frequency response is highly linear The FMDemodulation section consists of Phase Lock Loop (PLL) IC which demodulatesthe FM signal. This demodulated signal is then fed to the filter section to get theexact original signal.FM TRANSMITTER: The audio oscillator supplies the information signal andcould, if we wish, be replaced by a microphone and AF amplifier to providespeech and music instead of the sine wave signals.



MICMIC IN

AUDIO

IN

INTENSITY P3

D

R

I

V

E

R

TRANSMITTER RECEIVER

D

E

T

E

C

T

O

R

ANAL OG

OUT

JP 2

JP 3

+5V

+12V

JP 5 JP6 JP 7

TX2TX2

TX1TX1

FM

IN

FM MODULATOR

FM

OUT

IN

ANALOG

BUFFER

OUT TX IN

TX 1

FIBER

OPTICCABLE

RX 1

FM

DEMOD

IN

FM DEMODULATOR

PLL

FM

DEMOD

OUT

INOUT AUDIO

OUT

P5

VOLUME

SPEAKER

VCO

FILTER

BLOCK DIAGRAM FOR FREQUENCY MODULATION & DEMODULCATION USING VOICE LINK

OFF

SF1

ONSWITCH FAULTS

SF2

TX IN VI

S3

PROCEDURE:

Connect the power supply cables with proper polarity to kit. Whileconnecting this, ensure that the power supply is OFF.

Keep all the switch faults in OFF position. Connect the microphone provided with the kit to the socket marked MIC

on the kit. Connect AUDIO IN post to IN post of FM Modulator Section. Keep Jumpers JP2 towards +12V position, JP3 towards sine position, JP5

& JP6 towards TX1 position & JP7 shorted. Keep switch S3 towards TX IN position. Connect FM OUT post section of FM Modulator section to IN post of

Analog Buffer Section Connect OUT post of the Analog Buffer Section to TX IN post of

TRANSMITTER. Slightly unscrew the cap of LED SFH 756V TX1 (660 nm) from kit. Do not

remove the cap from the connector. Once the cap is loosened, insert thefiber into the cap and assure that the fiber is properly fixed. Now tight theap by screwing it back. Keep INTENSITY pot P3 at minimum position i.e

fully anti clockwise. Connect the other end of the fiber to detector SFH 250V (RX 1) in kit very

carefully. Observe the output signal from the detector at ANALOG OUT post on

CRO by adjusting INTENSITY pot P3, & you should get the reproductionof the original transmitted signal.

Connect ANALOG OUT in Receiver Transimpedence Amplifier Section toFM DEMOD IN post of FM Demodulator Section.



Connect FM DEMOD OUT post to IN post of Filter Section. Connect OUT post in Filter Section to AUDIO OUT post . Connect speaker provided with the kit to the socket marked SPEAKER on

the kit. Speak from the Microphone and you could hear your sound from the

speaker. Adjust INTENSITY (Optical Power Control) Pot P3 and VolumePot P5 to setup fiber optic audio link.

study of frequency modulation and demodulation using voice link

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